Considerable interest has recently been focused on the problem of limiting the mass of particulate matter emitted with the exhaust gases from diesel and other internal combustion engines. In the case of diesel engines, a great deal of effort is currently being expended to develop practical and efficient devices and methods for reducing emission of particulates in exhaust gases.
One method for accomplishing this is to provide a suitable particulate trap in the exhaust system of a diesel engine, the trap having at least one filter positioned therein which is capable of efficiently trapping the particulates from the exhaust gases and which is also adapted to be regenerated as by the in-place incineration of the trapped particulates collected thereby.
A ceramic wall-flow monolith particulate filter of the type disclosed, for example, in U.S. Pat. No. 4,364,761 entitled "Ceramic Filters For Diesel Exhaust Particulates and Methods of Making", issued Dec. 21, 1982 to Morris Berg, Carl F. Schaefer and William J. Johnston, has emerged as a preferred form of such a filter device.
Such a ceramic wall-flow monolith particulate filter includes an outer wall interconnected by a large number of interlaced, thin porous internal walls which define a honeycomb structure to provide parallel channels running the length thereof. Alternate cell channel openings on the monolith face are blocked and, at the opposite end the alternate channel openings are blocked in a similar manner but displaced by one cell whereby to define inlet channels and outlet channels.
With this filter arrangement, the exhaust gas cannot flow directly through a given inlet channel but is forced to flow through the separating porous walls into an adjacent outlet channel. The exhaust gas is thus filtered as it flows through the porous walls between adjacent channels.
As this type ceramic filter is presently manufactured, the ceramic walls thereof are fabricated by extrusion and then fired. After firing, the alternate channel openings are suitably sealed, as by being plugged with a non-porous material, to provide the structure described hereinabove with a plurality of inlet channels and a plurality of outlet channels arranged in checkerboard fashion.
Such a ceramic filter device is suitably located in the engine exhaust system of a vehicle so as to remove particulates from the exhaust gases by trapping of the particulates on the walls of the inlet passages or channels separating them from their associate adjacent outlet channels.
The filter will, with use, then become clogged with the carboneous material. The diesel particulates will increase the backpressure in the exhaust system of the diesel engine. It is thus necessary to remove the diesel particulates from the filter from time to time to prevent the deleterious effect on engine performance due to high backpressure.
The carboneous particulates, thus collected, can be removed from the filter by raising the temperature of the inlet gas to the particulate ignition temperature to effect incineration thereof. The carboneous particulate when produced from normal diesel fuel, that is, D-2 diesel fuel, will ignite if the temperature is raised to approximately 600.degree. C. in the presence of 15%-18% oxygen.
However, as is well known, a diesel engine achieves exhaust temperatures of this magnitude only under very severe engine loading conditions. Therefore, a supplementary source of heat to rise the exhaust inlet temperature to the wall flow ceramic filter is normally necessary. This usually requires the use of a relatively costly heat source, such as a fuel burner or an electrice resistance heater, in series with the exhaust flow to raise the temperature of the gases to approximately 600.degree. C.
It is also known in the art, that fuel additives, such as copper napthtenate, copper acetate, tetraethyl lead and manganese (MMT), in the diesel fuel will reduce the ignition temperature of diesel particulates to approximately 320.degree. C.-420.degree. C. The quantity of the additive content in the fuel has normally been from about 0.05 gm/gal to 0.75 gm/gal to effect this desired reduction of ignition temperature of the particulates.
It is also known that the particulates, from such treated diesel fuel, that are deposited on a fairly low heat conductive surface, whether it is metallic in nature or ceramic, can be ignited in a small area, for example, less than 1 mm diameter, by glowing engine sparks, electric arc, or a small pin point torch type fuel burner. After ignition, the particulate (using the above described metallic additives in the fuel) buring will readily propagate over those sufaces of the filter on which the particulates have been deposited.
As described above, the fuel additive normally will reduce the ignition temperature of the particulates to as low as 320.degree. C. depending on the additive and concentration used. In addition, the burning of a small portion of the particulate causes continuous layers to ignite and the combustion propagates. It is presumed that the metallic additive after having been exposed to the engine combustion process is throughly oxidized. These oxidized metallic particulates are throughly dispersed in the carboneous particulates. When the temperature of this dispersion is locally raised in temperature, then an exothermic reaction occurs. The oxygen molecules in the metal oxide freely combines with the carbon to form CO and CO.sub.2. The reaction produces a large quantity of energy and increased gas temperature which causes continuous ignition of the surrounding layers of the metal oxide-carbon dispersion.
When a wall flow ceramic monolith filter is to be regenerated, it is thus desirable, from a cost standpoint, to ignite only a small area of the face of the monolith. However, a conventional ceramic type filter monolith, with plugs of non-porous material, is not operative so as to permit particulates to collect on the inlet face thereof. Instead the particulates are only collected on the walls of the filter inlet channels. In order to insure complete regeneration, it is almost normally necessary that each channel be ignited on an individual basis. This latter statement is not entirely true in fact, because spot ignition in several inlet channels can and will propagate from channel to channel due to the heat generated in the long passages. However, this propagation generally occurs only toward the rear or discharge end of the filter and, accordingly, the front or inlet end portion of filter will still contain unburned particulates.